1. Field of the Invention
The invention relates in general to estimating three-dimensional location of a communications device.
2. Description of the Related Art
Positioning services have become very popular in the recent years. Positioning refers here to determining the location of a communications device. The communications device may be capable of determining its position based on signals it receives. The signals can be sent either from a specific positioning system or, for example, from a cellular communications system. Alternatively, the communications device may act as a measurement device and send measurement results to a further unit, which then determines the location of the communications device.
Positioning services may be used simply for locating the communications device. The location of the communications device may, for example, be shown on a map at the display of the communications device. Alternatively, it is possible to provide location-dependent services, for example, for users of a communication system. The location of the communications device may affect the content of a location-dependent service. A further option is that the location of the communications device is used for determining whether the service is provided to the communications device at all.
The most widely used positioning system is the Global Positioning System (GPS). GPS positioning is based on measuring relative time of arrival of signals sent simultaneously from a number of GPS satellites. The location of a GPS receiver is determined by determining the distance between GPS satellites and the GPS receiver using time of arrival measurement results together with the exact GPS time. The locations of the GPS satellites can be determined based on navigation information carried in the GPS signals. The navigation information includes both data unique to the transmitting satellite and data common to all satellites. The navigation information contains time information, satellite clock correction data, ephemeris (precise orbital parameters), almanac (coarse orbital parameters), health data for all satellites, coefficients for the ionospheric delay model and coefficients to calculate the Universal Coordinated Time (UTC) from the GPS system time.
In theory, three time of arrival measurements would be enough to calculate the GPS receiver's position and also the velocity, if the exact GPS time is known to the GPS receiver. In practice, a GPS receiver has low-cost, low-accuracy local oscillator as a local clock. Therefore a fourth time of arrival measurement is needed to determine the difference between the local time and the GPS time.
GPS positioning is dependent on obtaining accurate GPS time and navigation information and on performing the distance measurements. For carrying out GPS positioning successfully, signals from three or four GPS satellites need to be received properly for demodulating navigation data needed for the distance measurements. For shortening the time-to-first-fix, it is possible to provide information about the rough location of the GPS receiver to the GPS receiver. Assisted GPS (A-GPS) refers to sending some assistance information to a GPS receiver for helping the receiver to locate itself based on the GPS signals. One example of the assistance information for GPS is information on the rough location of the GPS receiver. By providing a rough estimate of the location of the GPS receiver as location assistance information, the response time of GPS positioning can be shortened.
A communications system, for example a cellular telecommunications system, may be used for transmitting the location assistance information to a communications device incorporating, in addition to the functionality needed for communicating with the communications system, a GPS receiver for receiving GPS signals. The communications system may optionally be equipped with a plurality of reference GPS receivers for obtaining other assistance information than only the rough estimate of the GPS receiver.
In connection with location a mobile station (a communications device) of a cellular telecommunication system, mobile-based positioning refers to determining the location in the mobile station based on the signals received from the cellular telecommunications network or from an external system, such as from the GPS. Mobile-assisted positioning refers to the cellular telecommunication network determining the location of the mobile station, but using information sent by the mobile station. MS-based A-GPS, for example, thus refers here to a location method, where location assistance information is sent to a communications device and the communications device determines its location using the location assistance information and the measurements it has performed on received GPS signals. On the other hand, MS-assisted A-GPS refers here to a location method, where location assistance information is sent to a communications device, which then performs the measurements on the received GPS signals and sends the measurements back to the telecommunication network, which finally uses such measurements for determining the device location. Both in MS-based and in MS-assisted A-GPS, the communication device uses location assistance information to shorten the time needed to perform the measurements on the received GPS signals.
As mentioned above, locating a communications device is possible also using measurements relating to signals of a communications system. Rough location estimates for a communications device can alternatively be obtained based on the identities of the cell(s), via which a communications device is currently communicating.
Until now the development of cellular network based LoCation Services (LCS) algorithms has been mostly focused towards algorithms capable of performing location calculations in a two-dimensional scenario. In other words, these algorithms assume that the Earth's surface locally flat, and they do not take into account the altitude fluctuations.
Algorithm developers have devoted little attention to location calculation algorithms capable of estimating the location of a communications device in a tri-dimensional scenario because of a lack of a clear need for such algorithms. Furthermore, estimating a location of a communications device in tri-dimensions gives rise to technical challenges such as the increased complexity of full tri-dimensional location calculation algorithms and the need of additional measurements for estimating the third coordinate. Recently, however, the results of cellular network based LCS algorithms have been considered to be a valuable component of the location assistance data to be provided to a GPS receiver for MS-based or MS-assisted A-GPS location method.
However, since the results of cellular network based LCS algorithms shall be represented in three-dimensions, a need has arisen for a feasible network based method for estimating the location of a communications device in three-dimensions.